If the little rubber part Mercedes only sells it as part of a whole new cap, a bit over $100 I'm afraid. However, there is a replacement you can use with a minor, imperceptible modification, you'll see it in the Help/DIY stickies in the W210 forum.

Take care and enjoy the ride,
Greg

Socialism is a philosophy of failure, the creed of ignorance, and the gospel of envy; its inherent virtue is the equal sharing of misery. (Winston Churchill)